The general objective of this project is to establish the mechanisms that determine how a nerve cell becomes connected to its particular synaptic targets during neurogenesis. On the basis of our observations on the visual system of Daphnia, we have proposed a spatio-temporal model in which particular cellular interactions underlie the formation of the specific pattern of synaptic connections in this system. We propose to extend our observations by producing specific perturbations in the developmental process in order to test the validity of this and other models for the formation of ordered synaptic connections in visual systems. Our principal approach is to use an ultraviolet microbeam to delete specific small groups of neurons at particular developmental stages. We will also explore the possibility of using sublethal dosages of radiation as a means for delaying the maturation of specific groups of neurons. In addition, 3H-thymidine incorporation and autoradiography will be used to determine the birthdates of the various neurons. The analysis of these experiments will be conducted at the morphological level by serial-sectioning the tissue for light and electron microscopy. Computer techniques for the 3-D reconstruction of cellular architecture will be employed in this analysis in order to ascertain in detail (a) the gross morphological effects of these lesions, (b) the number and location of cells deleted, (c) the morphology of individual neurons and their synaptic connectivity in experimental specimens, and (d) which specific neurons show a high density of silver grains following exposure to 3H-thymidine and autoradiography. The inherent capacity of our methods to produce and detect small changes at the cellular level gives us a unique opportunity to define and clarify the significance of cellular interactions to the formation of nerve connections.